This invention relates to a method for manufacturing magnetic recording media, and more particularly to a method for preparing a magnetic solution by dispersion, the magnetic solution being applied to a support.
Methods for manufacturing magnetic recording media such as magnetic tapes and floppy disks are well known in the art in which a magnetic solution prepared by dispersing ferromagnetic particles is applied to a belt-shaped support which is being conveyed continuously in the longitudinal direction, and in which the magnetic solution on the support is magnetically oriented and dried. The resultant support is cut or stamped out to form a magnetic recording media.
The magnetic solution applied to the support is generally prepared by kneading and dispersing ferromagnetic particles, a bonding agent, a dispersing agent, and a lubricant with an organic solvent. The dispersion of the ferromagnetic particles in the magnetic solution thus prepared greatly affects the electromagnetic conversion characteristic of the magnetic recording medium which is formed through the above-described steps of applying the magnetic solution to the support, magnetically orientating the magnetic solution on the support, and then drying the magnetic solution.
Recently, there has been a strong demand for increasing the magnetic particle density of a magnetic recording media. To meet this requirement, ferromagnetic particles having a small grain size and a large specific surface area are used to improve the S/N ratio and squareness ratio. It is essential to uniformly disperse the ferromagnetic particles in the bonding agent.
Much research has been carried out regarding the dispersion and magnetic orientation of .gamma.-Fe.sub.2 O.sub.3 particles for magnetic recording media to determine the optimum method. A main issue for the characteristics of a magnetic recording medium concerns the effect of the mutual action between .gamma.-Fe.sub.2 O.sub.3 particles and macro molecules and the behavior of adsorption of macro molecules on the dispersion and magnetic orientation of the .gamma.-Fe.sub.2 O.sub.3 particles. Accordingly, the dispersion of ferromagnetic particles in a magnetic solution must be evaluated. For this purpose, a rotational vibration method has been proposed by Katsuhiko Nakamae et aI. (see "Hyomen" (Surfaces) vol. 25, no. 7 (1987), page 401). In this method, a low magnetic field is applied for a certain time period in which the magnetic moment is not inverted by magnetization, and the value M.sub.H /M.sub.S, which is obtained by dividing the magnetization M.sub.H of the magnetic solution at that time by the saturation magnetization M.sub.S, is employed to evaluate the dispersion of ferromagnetic particles in the magnetic solution.
On the other hand, it has been found through the application of the above-described evaluation method to magnetic solutions that, even with the same kind of ferromagnetic particles, the value M.sub.H /M.sub.S depends greatly on the prescribed magnetic solution, particularly, on the viscosity thereof. That is, even when magnetic solutions are selected which have equal dispersion states, a magnetic solution having a high viscosity has a low M.sub.H /M.sub.S value, and conversely a magnetic solution having a low viscosity has a high M.sub.H /M.sub.S value. This effect of viscosity is neither referred to nor addressed in the aforementioned reference, and likewise neither is the value M.sub.H /M.sub.S for the actual magnetic solutions. Hence, it is difficult to apply the evaluation method to an actual method for manufacturing a magnetic solution.
To obtain a magnetic solution having a high dispersion, a method for evaluating a magnetic solution must be established. However, there has not yet been proposed a method for evaluating the dispersion of a magnetic solution with high accuracy.